I think we can know that through their reasons, the consistency of their action, their reaction to their own behavior, and I do believe that it's important to know the environment or people they live with too. The definition of bad is different for some people. They may not realize what they do is considered bad to others. It can be tricky to see the consistency of their action if they're used to live in an environment or live with people who like to do something bad and so their judgement of doing something bad is not that clear or obvious to them.

So to me, personally, the two things we can do at first are talking and observing them directly because we should not judge a person easily.